Automotive power transmission fluids are called upon to provide specific frictional properties under very demanding conditions of temperature and pressure. Changes in a fluid's frictional properties as a function of relative sliding speed, temperature, or pressure as a result of these conditions may cause performance degradation immediately noticeable to the vehicle operator. Such effects may include unacceptably long or short gear shifts, vehicle shudder or vibration, noise, and/or harsh shifts (“gear change shock”). Thus, there is a need for transmission fluids that undergo minimal frictional changes under conditions of high temperatures and pressures. Such fluids would minimize equipment and performance problems while maximizing the interval between fluid changes. By enabling smooth engagement of torque converter and shifting clutches, these fluids would minimize shudder, vibration, and/or noise, and in some cases improve fuel economy, over a longer fluid lifetime.
Friction modifiers are used in automatic transmission fluids to decrease friction between surfaces (e.g., the members of a torque converter clutch or a shifting clutch) at low sliding speeds. The result is a friction vs. velocity (μ-v) curve that has a positive slope, which in turn leads to smooth clutch engagements and minimizes “stick-slip” behavior (e.g., shudder, noise, and harsh shifts). Many conventional organic friction modifiers, however, are thermally unstable. Upon prolonged exposure to heat, these additives decompose, and the benefits they confer on clutch performance are lost.
EP 0 020 037 discloses an oil-soluble friction-reducing additive which may be used in a crankcase of an internal combustion engine. The additive may be added to a lubricating oil in an amount of from about 0.05 to about 3 weight percent to form a motor oil. The additive may also be used in fuel compositions, such as diesel fuel and gasoline. The fuel composition may comprise from about 0.001 to about 0.25 weight percent of the additive.
Reissue Pat. No. 34,459 discloses a friction reducing additive which may be present in a lubricant composition in an amount of from about 0.1% to about 2.0% by weight. The lubricant composition may be used in a wet brake system.
U.S. Pat. No. 5,126,064 discloses a lubricant composition comprising from about 0.25 to about 15% of at least one succinimide derivative. Gear oil blends 2, 4, and 6 are disclosed which comprise 0.5% or 0.25% by weight of a succinimide friction modifier. The lubricant composition may be used to reduce the noise generated by slipping clutch plates during the operation of a limited slip differential.
U.S. Pat. No. 5,767,045 discloses a hydraulic fluid comprising from about 0.03 to less than 1% of a C18-C24 alkenyl succinimide.
U.S. Pat. No. 5,225,093 discloses an additive comprising from about 10 to about 80% of an oil-soluble succinimide. The additive may be used in compositions for use in manual transmission oils and in gear oils, such as rear axle lubricants.
U.S. Pat. No. 5,942,470 discloses gear oils comprising at least one oil-soluble succinimide in an amount of from about 0.05 to about 4% by weight. The gear oil additive concentrate may comprise at least one oil-soluble succinimide in an amount of from about 1 to about 20% by weight.
U.S. Pat. No. 6,096,691 discloses an oil-soluble top treat additive concentrate comprising from 10 to 30% by weight of at least one 3-hydrocarbyl-2,5-diketopyrrolidine. The patent also discloses a gear lubricant composition comprising from 0.06 to 4% by weight of at least one 3-hydrocarbyl-2,5-diketopyrrolidine.
Power transmission fluids formulated according to the present disclosure may provide at least one of improved friction durability, improved performance for smooth engagement of torque converter and shifting clutches, may minimize shudder, vibration and/or noise, and/or improve fuel economy. The power transmission fluids may provide improved friction durability, i.e., friction characteristics that change very little when the fluid is subjected to thermal and oxidative stresses.